This invention relates to a cylinder cabinet piping system for a gas cylinder which supplies a feed as to a thin-film making or fine pattern etching process, more particularly to a cylinder cabinet piping system which makes it possible to form high-quality thin films and to conduct high quality, fine pattern etching.
Recently, a process for forming high-quality thin films or high quality, fine pattern etching requires a technique that supplies a superhighly pure gas to such a process to keep the process environment highly pure.
Taking a semiconductor device as an example, the unit elements become more and more small in size to enhance the degree of integration of an integrated circuit, and extensive research and development programs are being carried out to commercialize a semiconductor device of micron or submicron order, or even of 0.5 .mu.m or less. Such devices are produced by repeating a thin-film making process and a process for etching these thin films into given circuit patterns, where a silicon water is normally placed in a reaction chamber filled with a given gas. The reaction chamber is kept under vacuum to obtain a longer mean free path of the gas molecules for etching and filling through holes and contact holes of a high aspect ratio, and also to control the vapor-phase reactions. Impurities present in these processes cause several problems, such as degradation of the thin films, lowered accuracy of the fine structures, and the insufficient adhesion between the thin films. It is essential to have the reaction environments for film making and etching fully controlled, in order to produce integrated circuits of submicron or lower submicron patterns, densely on a large-size wafer at a high yield. This requires a technique that can supply a superhighly pure gas to the process.
The gases for semiconductor production may broadly fall into two general categories; one includes relatively stable, common gases such as N.sub.2, Ar, He, O.sub.2, and H.sub.2, and the other are special gases which are either strongly toxic, spontaneously combustible or corrosive, such as AsH.sub.3, PH.sub.3, SiH.sub.4, Si.sub.2 H.sub.6, HCl, NH.sub.3, Cl.sub.2, CF.sub.4, SF.sub.6, NF.sub.3 and WF.sub.6. Those gases falling into the first category are mostly sent directly from the purification unit to the semiconductor production unit, since they can be handled relatively easily. The storage, purification and piping systems have been sufficiently developed to directly supply these gases from the purification to the process unit, (Tadahiro Ohmi, "Challenges to ppt, gas piping system for semiconductor production, which challenges to ppt impurities", Nikkel Micro-devices, pp. 98-119, July 1987). Those gases falling into the second category, on the other hand, are mostly sent from cylinders to the process unit via a cylinder cabinet piping system, since they must be handled much more carefully and are used in smaller quantities.
The most critical problems associated with the gas supply from the cylinder to the process unit via cylinder cabinet piping system are coming from contamination of the gases caused by stains on the internal walls, leaks at the valve-cylinder connections and large quantities of gases adsorbed on the cylinder valve internal walls which permits no cleaning. These problems, however, have been mostly solved by the application of combined electro-polishing to the internal walls to make them into a mirror surfaces free of fabrication-related degradation, and the development of an external screw type cylinder valve in which an MCG (Metal C ring Fitting), incorporating a purge valve, is used.
(Tadahiro Ohmi and Junich Murota, "Clean Cylinders and Gas Filing Technology", 6th Super LSI Ultraclean Technology Symposium Proceedings, and "High-Performance Process Technology III", pp. 109-128, January, 1988). These developments, however, are still incapable of coping with contamination of the purge gas with moisture released from the piping internal walls, since there are a number of gas stagnant sections in the purge gas system in the cylinder cabinet piping system. The gas supply line extending from the cylinder valve to the process unit will be contaminated with moisture when purged with the gas containing a large quantity of moisture. This contamination could eventually damage the process by lowering the purity of the feed gases to the process, even when they themselves are sufficiently clean in the cylinders.
For example, the newly developed DC-RF coupled bias sputtering device gives excellent, mirror-surfaced Al thin films which are completely free of hillocks, even when they are thermally treated at 400.degree. C. (T. Ohmi, H. Kuwabara, T. Shibata and T. Kiyota, "RF-DC coupled mode bias sputtering for ULSI metalization". Proc. 1st. Int. Symp. on Ultra Large Scale Integration Science and Technology, May 10-15, 1987, Philadelphia, and T. Ohmi, "Complete Removal of Impurities, Grasp of Hillock-Free Al Film-Making Conditions", Nikkei Microdevices, pp. 109-111, October, 1987). In the production of Al thin films using the above device, the presence of 10 p.p.b. or more, moisture in the Ar sputtering atmosphere will degrade the surface morphologies of the Al thin films. It is therefore impossible, when the moisture is present beyond the above level, to optimize the film-making conditions for production of the hillock-free Al thin films whose surface resisticity is the same as that of the bulk Al body. It has been found that the Al film-making conditions can not be optimized unless the moisture level is lowered below 10 ppb.
It has been further found that vaccum CVD, which was before considered to be incapable of growing a Si thin film selectively and epitaxially under the commercial conditions (650.degree. C. and several Torr.), allows the selective and epitaxial growth, provided that the moisture adsorbed on the wafer is sufficiently controlled by using superpure SiH.sub.4, H.sub.2, and N.sub.2 gases containing moisture at 10 ppb or less. Thus, Si can be epitaxially grown on a clean Si surface, while growth of polysilicon on SiO.sub.2 is efficiently controlled (Junichi Murota, Naoto Nakamura, Manabu Kato, Nobuo Mikoshiba and Tadahiro Ohmi, "Highly Selective, Ultraclean CVD Technology", 6th Super LSI Ultraclean Technology, III", January, 1988).
FIG. 14 presents a flow of one of the most well-arranged, conventional cylinder cabinet piping systems. A cylinder cabinet piping system has, in general, 3 to 6 gas cylinders, and 100 to 200 units are in service for semiconductor production lines in a semiconductor factory.
A cylinder cabinet piping system is described by referring to FIG. 14. This system has, for simplicity, two gas cylinders 101 and 102, each provided with a cylinder valve 103 or 104 consisting of a main valve and purge valve; 107 and 108. 109 and 110 are spiral pipes to provide a flexibility for the piping system for easily removing the cylinder from the system, which are normally 1/8" size stainless pipes (SUS316L) electropolished for their internal walls; 111 and 112 are 3-port branching valves where only one port is opened or closed; 113, 114, 117, 118, 121, 122, 125, 126, 129, 130, 131, 132, 133, 134, 135, 136, 137 are stop valves; 115 and 116 are filters to remove foreign matter from the gases; 119 and 120 are pressure regulators; 123 and 124 are emergency shut-off valves for emergencies such as earthquakes; 127 and 128 are check valves which provide a flow only in the direction indicated by the arrow; 145 is a vacuum gauge; 138 is a large-size (1/2" or 1") vacuum exhaust line of stainless steel; 139 and 140 are purge lines to purge the cylinder cabinet piping system, where N2 and Ar flow through 139 and 140, respectively; 105 and 106 are feed supply lines to pass the feed gases in the cylinder to the process unit. The purge and feed supply lines are of stainless steel (SUS316L), 1/4" or 3/8" in size depending on the quantity of the gases flowing therethrough, the inner walls of which are electro-polished. 146 is a supporter for the cylinder cabinet piping system.
Next, the functions of each component and the operational procedures are described, by referring to FIG. 14, for one system line for simplicity. First, the procedures for replacing the cylinder are described. The cylinder cabinet piping system is contaminated more on this occasion than on any other by in-flowing external atmosphere. The gas cylinder 101 is removed out of the system, with the main and purge valves of the cylinder valve 103 open, the stop valves 113 and 137 closed, the branching valve open and the stop valve 129 and 136 open. The system is contaminated to a lesser extent while the cylinder is being removed, since N.sub.2 gas from the N.sub.2 purge gas line flows outward after passing through the spiral lines 107 and 109. Velocities of the N.sub.2 as flowing out of the system are normally set at 3 to 10 m/s. The higher the velocity at which N.sub.2 flows out, the less the extent of the system contamination. The new cylinder is mounted on the system, while N.sub.2 gas is being blow. On completing the cylinder replacement, the N.sub.2 gas remaining in the system is evacuated with the purge valve of the cylinder valve, the valve 129 and the valves 113, 131, 133 and 135 open, and the branching valve 111 and the valve 117 closed. The vacuum pump used is a turbomolecular pump free of oil contamination by the reverse diffusion. Vacuum exhaustion through small-size pipes is insufficient to completely drive off the N.sub.2 contaminated with trace components of atmosphere in one time. Next, the system is filled with Ar to a given pressure level (for example, 2 to 5kg/cm.sup.2), with the valve 137 open and the valve 133 closed, and then the Ar gas is again evacuated with the valve 137 closed and 133 open. The Ar filling/evacuating is repeated 5 to 10 times before the feed gas is supplied to the feed supply line. Then, the feed gas supply is started by closing the purge valve of the cylinder valve and the valves 129, 137 131 and 135, and opening the valve 121 and finally the main valve of the cylinder valve. Ar is used as the batch purge gas, because it is inert, to have no effect on the process even when it remains in the system in small quantities, easily purified, and relatively inexpensive.
As described above, the gas in the purge lines 139 and 140 remain stagnant while the feed gas is being supplied from the cylinder 101 to the supply line 105. This period is very long, since a semiconductor production line frequently operates continuously for 24 hrs. When the purge gas is left stagnant for a long time, its purity is invariably lowered by moisture or other gaseous impurities released from the pipe inner walls. The purge line system, having a number of branches, is more affected by these impurities. When 2 or 3 branches are in service, the purge gas flowing therethrough is so small in quantity that most parts of the system are left essentially intact. As a result, the purity of the purge gas is lowered.
The lower-purity purge gas remaining in the system hinders greatly the semiconductor production, since it contaminates the gas supply line during the next purging period. Taking moisture as an example, which poses critical problems in the semiconductor production, moisture in the purge gas supplied from a gas supply line in a cylinder cabinet piping system was measured 2 ppb when the purge gas was allowed to flow continuously through the entire system, but this level increased to 30 and 80 to 90 ppb when the purge gas was blown at 6l/min. and 1 l/min., respectively, while all the branches, after having being sealed for 1 month, were left stagnant. The system used in the above tests comprised a 3/8" size, about 30m long, stainless steel pipe (SUS316L) serving as a Ar purge gas line extending from an Ar purification unit to a cylinder cabinet piping system, and 3/8" size, stainless steel (SUS316L) branching pipes leading to other units having a total length of about 34 m and 6 branching points, where the inner walls of these purge gas line and branch lines were electropolished. As shown, water content in the purge gas increased, to a varying extent depending on the purge gas rate, due to the presence of stagnant sections in the system. This resulted from contamination of the purge gas with the stagnant gas to which gaseous impurities were released from the inner walls. Purity of the purge gas decreases in such a cylinder cabinet piping system as that shown in FIG. 14, due to stagnation of large quantities of gases in the purge gas line and the branches therefrom.
It is thus essential to develop a piping technique that can eliminate stagnant sections from the entire system including the branch systems, in order to supply a superhigh-purity cylinder gas to a semiconductor production unit. Before the present invention, no such piping technique has been developed that prevents stagnation of purge gas line in a cylinder cabinet piping system.
In addition, those techniques are also in demand that can effectively prevent contamination of each line with air or a gas from another line, in order to keep the feed gas superhighly pure.
The other problems involved in the conventional cylinder cabinet piping system are those associated with airtight or leak tests. Airtightness of the connections around a newly mounted cylinder is confirmed either by a bubble test or pressure drop test after sealing a high pressure gas in the space to be tested. Quantity of leak detectable by these tests, however, is limited to 10.sup.-3 to 10.sup.-6 Torr l/s. The piping fittings are so developed at present as to make it possible to decrease a leak quantity to 2.times.10.sup.-11 Torr l/s or less, and the airtight test for such a system is carried out under a vacuum state using a He leak detector. It is thus to be desired that a cylinder cabinet piping system has connections around the cylinder so structured as to be suited for such airtight test, in order for it to supply a superhigh purity as to a process unit.
It is the object of the present invention to provide a cylinder cabinet piping system, which has no stagnant sections in the purge gas line, prevents contamination of each line with air or gas from another line as far as possible and has connections around the cylinder so structured as to be suited for a high-precision airtight test, in order to solve the above problems.